The present invention relates to an exhaust gas heat exchanger having stacked flat tubes such as an EGR cooler, in which boiling of cooling water inside a case is suppressed.
In order to reduce nitrogen oxide (NOx) contained in exhaust gas exhausted from an engine of a vehicle or the like, or to reduce pumping loss, to mount an EGR (Exhaust Gas Recirculation) device on a vehicle is generally performed. In many cases, in order to lower combustion temperatures in the engine, for this EGR device, an EGR cooler, which is a kind of an exhaust gas heat exchanger and for cooling the exhaust gas, is provided, in a line through which a part of the exhaust gas is recirculated to an intake side of the engine.
A general EGR cooler includes a stacked tube body arranged inside a case; the cooler configured such that exhaust gas flows in from one end part of a stacked tube body in a tube axis direction to circulate through the inside of respective flat tubes and flow out from the other end part; and cooling water introduced from a cooling water introduction part provided for the case is supplied to the above-described one end part and circulates through an outer surface side of respective flat tubes.
In the EGR cooler configured as described above, exhaust gas having flowed in from one end part of the stacked tube body in the tube axis direction is cooled, while circulating through the inside of respective tubes and flowing out from the other end part, with cooling water circulating in the same direction as the exhaust gas through the outer surface side of the tube. The exhaust gas in the EGR cooler has the highest temperature at a part at which the exhaust gas flows into the stacked tube body (one end part of the above-described stacked tube body in the tube axis direction), and the temperature gradually falls due to heat exchange with the cooling water while the exhaust gas circulates through the inside of respective tubes to become the lowest at the part where it flows out from the stacked tube body (the other end part of the above-described stacked tube body in the tube axis direction).
However, usually, a cooling water introduction part is provided at a corner part on one side of a case, and cooling water that flows in from the introduction part and flows through a gap between respective tubes causes easily drift in which the cooling water flows disproportionately to a part with low flow resistance rather than a part with high flow resistance and tends not to be distributed evenly to a cooling water inflow part of respective tubes. In general, presence of difference in distances from a cooling water introduction part provided at a corner part on one side of a case to each position of cooling water inflow parts of a stacked tube body is a main factor of the difference in flow resistances. Then, when viewed from the entire stacked tube body, temperature in a part of a stacked tube body near the inflow part of exhaust gas is made high, and cooling water in a part in which a flow quantity is reduced due to drift easily generates in particular local boiling.
In order to suppress such local boiling of cooling water, installation of a cooling water supply chamber, which has an effect of causing cooling water distribution to a stacked tube body to be uniform, is proposed. For example, in Patent Literature 1, a device is disclosed, in which an annular cooling water supply chamber is externally mounted on one end part of a peripheral wall of a case and an inlet tube is connected to the cooling water supply chamber, and, in addition, an annular slit hole interconnecting the inside of the cooling water supply chamber and the inside of the case is oriented toward a case part inside the cooling water supply chamber.
Moreover, in Japanese Patent Application Laid-Open Publication No. 2005-69064, installation of a cooling water supply chamber having a shape different from that in the Japanese Patent Application Laid-Open Publication No. 2007-154683 is disclosed. In the cooling water supply chamber in Japanese Patent Application Laid-Open Publication No. 2005-69064, the tip part thereof is connected to a cooling water inlet tube, and the end part is interconnected to a case housing a stacked tube body. Width of the cooling water supply chamber is gradually expanded from the cooling water inlet tube side toward the case side, and the expanded end part coincides with a case width of a part housing the stacked tube body. Consequently, it is so configured that cooling water can be supplied uniformly over the entire case width.